Reactions involving a single solid phase

Formation and growth of nuclei have been observed microscopically in single crystals in several decomposition and dehydration reactions. 

Thermal decompositions are sensitive to the nature of pretreatment given to solids. Perhaps the earliest observation in this respect is that of Michael Faraday who noticed that the efflorescence of sodium carbonate was promoted by scratching the crystal with a pin. Modern interpretation of this observation is that dislocations produced by scratching provide sites for the reaction. Enhanced reactivity at dislocations has been demonstrated in several decomposition reactions (Thomas Williams, 1971 Parasnis, 1970). Preirradiation of solids with neutrons, protons, or with UV-, X- or y-radiation decreases the induction period and increases the rate of the decomposition without affecting the activation energy significantly. This observation has been taken to 

When the reactivity of a solid is controlled by the crystal structure, rather than by the chemical constituents of the crystal, the reaction is said to be topochemically controlled. The nature of products obtained in a decomposition reaction is frequently decided by topochemical factors, particularly when the reaction occurs within the solid without separation of a new phase (Thomas, 1974 Manohar, 1974). A topotactic reaction is a solid state reaction where the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Dehydration of Mo03-2H20 is a typical example of a topotactic reaction  

Decomposition of WO3, M0O3, Ti02, etc., yielding mixed-valence phases is another reaction of interest  

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